1. Field of the Invention
The present invention relates to a developing system and an image forming apparatus incorporating same, and more particularly, to a developing system that develops an electrostatic latent image on a photoconductive surface using developer, and an image forming apparatus incorporating such a developing system.
2. Discussion of the Background
In many electrophotographic image forming apparatuses, such as photocopiers, printers, facsimiles, plotters, or multifunctional machines with electrophotographic capabilities, two-component developers formed of toner and carrier particles are widely used to develop a visible toner image from an electrostatic latent image formed on a photoconductive surface.
Typically, a two-component developing system includes a developing process, which converts an electrostatic latent image into visible form using toner, and a replenishing process, which supplies new toner to the developer after use and mixes the replenished material for recirculation to the developing process. In such a configuration where the developer is reclaimed for repeated use, it is important to maintain a constant toner concentration and distribution and a constant electrical charge in the developer throughout the replenishing process, so as to achieve a stable quality of toner images produced by the developing process. For this purpose, a common replenishing process adjusts the toner concentration by supplying toner in an amount determined in proportion to the consumed amount, and subsequently mixes the developer with the toner supply to achieve uniformity of the resulting mixture, in which electrical charges are generated by friction between toner and carrier particles.
In a conventional developing system, the replenishment takes place immediately prior to the developing process, where developer is mixed and charged by rotating screw conveyors in a developer sump located close to a development roller that magnetically attracts the developer being mixed for immediate use in the developing process. When used in an environment with a high toner consumption/supply rate, the close interval between replenishment and development may result in insufficient mixing of the replenished developer, which eventually causes a loss of print quality, such as background smudging and/or toner scattering.
To enhance mixing of two-component developer, a developing system has been proposed having a separate replenishing unit and a developing unit connected by a pneumatic path. A common configuration of such a developing system includes a mixing container and a measuring feeder forming the replenishing unit, and a delivery tube and an air pump forming a pneumatic path that connects the replenishing unit to the separate developing unit.
In use, the mixing container mixes developer with new toner so as to obtain appropriate toner concentration and electrical charges therein as required by the material conditions. The measuring feeder feeds regulated amounts of developer from the mixing container to the pneumatic path, which delivers the particulate material to the developing unit using compressed air. In the pneumatic path, the air pump pressurizes air to generate a positive pressure in the delivery path relative to the developing unit and the mixing container which are in communication therewith and therefore are both under atmospheric pressure. The compressed air thus generated propels the developer from the pressure source to the developing unit along the delivery tube.
Occasionally, the pneumatic path in such a developing system suffers from leakage of compressed air where the delivery tube connects to the replenishing unit, i.e., a dispensing opening of the measuring feeder. Such air leakage naturally causes a reduction in propelling pressure leading to insufficient delivery performance, and the compressed air leaking into the mixing container obstructs the flow of developer from the mixing container to the measuring feeder, resulting in reduction or variation in a particle dispensing rate of the measuring feeder. It is therefore desirable to seal off the dispensing opening of the measuring feeder when there is compressed air flowing in the delivery tube.
One approach to achieving this objective is to provide a measuring feeder with sealing capability. Generally, a measuring feeder for feeding developer material is implemented using a rotary feeder formed of a multi-bladed rotor and a stator surrounding the rotor blades. Such rotary feeders can feed developer in a controlled and regulated manner, but often do not offer the reliable sealing required to prevent air leakage in the pneumatic delivery of particles. A good sealing may be provided by forming the rotor blades of resilient material to fit tightly in the surrounding stator, which, however, seems impractical because rubbing the rotor blades against the stator wall will eventually cause significant degradation of the metering mechanism. Moreover, the resilient blade configuration may not have satisfactory durability when used in a two-component developing system that handles hard carrier particles formed of iron and/or ferrite material.
Consequently, what is needed is a two-component developing system having a replenishing unit with a pneumatic delivery path, which can replenish developer with appropriate toner concentration and electrical charges and supply the replenished material for development in regulated amounts reliably and efficiently. An image forming apparatus incorporating such a developing system would achieve excellent electrophotographic performance with reliable and stable imaging quality.